Rebated boat tail. It's an interesting concept.
One advantage is that it offers better BC performance than flat base, yet allows the transition of the bearing surface out of the muzzle to be more consistent as well as providing the possibility of less upset during and just after emergence.
More appropriate to the OP question; the bullet length and intrusion into the powder column is (much?) reduced, putting a respectable BC into a more magazine tolerant length cartridge.
That's one of the key reasons why I'm trying to redevelop my loads to employ the
Speer Gold Dot Rifle Bullets, to eliminate or reduce charge compression while still maintaining a magazine length tolerant cartridge. They combine the rebated base with excellent reported accuracy as well as highly respectable terminal performance. This was precisely the advantage that I was seeking with the Hornady ELD-X; but I ran into pressure issues, and dropped the Hornady bullets from the initial project at that point.
Lockdowns and my age/health tend to preclude getting out to shoot right now; but this project is on the front burner for when conditions improve.
Wednesday, my brand new weather station reported 117* under the sun shade on my patio. My shop temp was 81* at 6:00 AM today. 80-85* is where my personal performance starts to lag. 100+* temps are forecast for the next week.
I am currently set up with test loads for 223 62gr, 223 75gr, 6.5G 120gr, 260 120gr, 260 140gr, 308 150gr, and 308 168gr. Where applicable, the testing loads are simply bullet swaps into already existing accurate loads, and where none exist; I'm starting with 90% of max according to the Speer reloading manual. No loads are being tested that exceed 95% of published max from that source. Initial loads are intended primarily as pressure tests to determine safe upper pressure levels; but they will be shot for accuracy as well.
FWIW, I agree that the HDY 75 ELD is not suitable due to powder column intrusion at magazine length, but also assert that the HDY 75gr HPBT Match has been my SR 300yd to MR 600yd goto bullet in 223 for around a decade.
FWIW, I consider the 223 to be best at distances out to 600yd, and look for more energetic chamberings to explore accuracy out beyond that. My usual MR-LR chamberings are 6.5G and 260 Rem. I have complete confidence in the 75gr Gold Dot and 75gr HDY HPBT Match projectiles in 223 out to 600yd, and no confidence
in myself beyond that point with the223. There are better chamberings for that.
IMHO, the issue with 'what's the best...?' questions is that they get asked in anticipation of stretches the imagination that go beyond practical limits. Taking chances is not a reasonable strategy for longer distance marksmanship.
Greg